1. Field of the Invention
The invention relates in general to a color liquid crystal display (LCD). More particularly, to a twisted nematic (TN) color liquid crystal display.
2. Description of the Related Art
Being affected by an external effect such as an electric field or thermal energy, the molecular arrangement of a liquid crystal is altered to cause variation in optical characteristics such as birefringence, optical rotatory power, dichroism, scattering and optical rotatory dispersion. The visual effect due to the variation of these optical characteristics can thus be utilized in the fabrication of liquid crystal display. The main structure of a liquid crystal display comprises a liquid crystal unit disposed between two glass substrates covered by transparent electrodes.
The nematic liquid crystal comprises bacilliform molecules arranged parallel to a long axis direction of each liquid crystal molecule. In addition, these liquid crystal molecules are mobile as a fluid. Nematic liquid crystal has a characteristic of anisotropy for optics, electricity, and magnetism. Being experienced an external electric field or magnetic field, the arrangement of these liquid crystal molecules is altered. As a consequence, the related characteristics of the liquid crystal are changed. However, while the electric or magnetic field is removed, the liquid crystal restores to an original arrangement before the alteration.
Twisted nematic liquid crystal display is one of those liquid crystal displays controlled by electric field. The main body comprises a liquid crystal unit and two polarizers. The liquid crystal unit further comprises two parallel glass substrates covered by transparent electrodes and alignment films. Therefore, a specific molecular alignment of the liquid crystal molecules on the alignment films is achieved. Between the glass substrates, the liquid crystal molecules may continuously twist the molecular orientation along a direction of the alignment film to obtain a twist angle of 90 degree. This is the so-called twisted nematic liquid crystal. In addition, the polarizers are respectively disposed on the top and bottom surface of the liquid crystal unit.
FIGS. 1A to 1B shows a schematic diagram for the operation of a convention twisted nematic liquid crystal display. In FIG. 1A, directions of the polarization for two parallel polarizers 100a and 100b are perpendicular to each other. The liquid crystal molecules 104 between a first glass substrate 102a and a second glass substrate 102b continuously twist the molecular orientation thereof according to the direction of the surface alignment films on the first and the second glass substrates 102a and 102b. Since the surface alignments on the glass substrates 102a and 102b are perpendicular to each other, an angle of the molecules between the first glass substrate 102a and the second glass substrate is 90 degree.
In FIG. 1A, the molecular alignments between the surface of the first polarizer 100a and the surface of the second glass substrate 102a are parallel, and the molecular alignments between the surface of the second polarizer 100b and the surface of the first glass substrate are 102b. Since the polarization axes of the polarizers 100a and 100b are perpendicular to each other, the twisted angle of the liquid crystal molecules is 90 degree, and the molecular alignment on each of the surface of the glass substrate is parallel to the polarization axis of the corresponding polarizer. Thus, while a light is incident onto the first polarizer 100a with a right angle, only the part of the incident light having an axis parallel to the polarization axis of the polarizer 100a can pass through to enter the liquid crystal unit. The polarization axis of the light entering the liquid crystal unit is then twisted along the orientation of the liquid crystal molecules with a right angle. This is the 90 degree optical rotation. Being twisted, the light passing through the liquid crystal molecules has a polarization axis parallel to that of the second polarizer 100b. The light can thus travel through the second polarizer 100b to display a bright image.
In FIG. 1B, by a voltage is applied on transparent electrodes on surfaces of the first and the second glass substrates 102a and 102b. The molecular alignment of the liquid crystal molecules 104 between the glass substrates 102a and 102b is changed to an arrangement parallel to the applied external electric field. Since the polarization axes of the polarizers 100a and 100b are perpendicular to each other, and the molecule alignment of the liquid crystal molecules 104 is parallel to the external electric field, the polarization axis of the incident light is parallel to that of the first polarizer without being affected by the liquid crystal molecules 104 parallel to the external electric field. After passing through the liquid crystal unit, the incident light retains a polarization axis which is parallel to the first polarizer 100a and perpendicular to the second polarizer 100b. Therefore, the light is blocked by the second polarizer 100b, and a dark image is displayed.
The twisted nematic liquid crystal display can be used as a black-and-while display by adapting the characteristic that the arrangement direction of the liquid crystal molecule is changed as an external electric field. To obtain a color display with the twisted nematic liquid crystal display, an external optical device such as a colored or multi-color external light source, a color polarizer, color filter, color reflector is used in a passive way. The additional colored or multi-color light source consumes an extra voltage load, so that the fabrication cost and the power consumption are increased. Using a color filter to achieve the color effect, a part of the light is absorbed by the color filter to reduce the light intensity. This is thus not suitable for use in the reflection type liquid crystal display equipment. Moreover, the fabrication cost is high, and the fabrication process is complex.